In European patent 825 450 a shielding device is described, i.e., an electromagnetic shielding device for nuclear magnetic resonance apparati, particularly for apparati intended for imaging specific body parts. The device has a detection cavity with one opening for introducing the body part under examination, an electromagnetic shielding structure enclosing transmitting and receiving coils, and removable shielding elements electrically connectable to said shielding structure, which removable elements have the function of reducing the span of the opening of the detection cavity and so to prevent or reduce the infiltration of electromagnetic noise from the outside to the inside of said cavity for acquiring MRI signals, particularly in apparati wherein the cavity is a tubular one i.e. it has at least an opening and/or two opposite coaxial openings, and wherein the rigid shielding element is a tubular element extending from one to the other one of the opposite openings.
These removable elements are composed of sleeves extending the tubular shielding element and can be compressed with a predetermined force against body parts under examination at least at one passage opening for the body part. In order to connect the patient body to the shield, and thus to the ground, and to neutralize possible electromagnetic noise said sleeves are composed of a conductive material layer on the inner side and at the opening contacting the body they have a coulisse wherein a string is housed for fastening the sleeve against the body.
While this device satisfactorily serves its function, it has some drawbacks.
MRI apparati used for imaging parts of the body, so called dedicated apparati, such as parts of arms or legs, are structures having small size wherein the detection cavity is defined in its shape substantially by the shape of the coil of the magnet generating the static field. In apparati of this type, wherein the generated magnetic field has a middle low intensity, it is important to reduce at a minimum infiltrations of electromagnetic noise inside the cavity itself.
Moreover in nuclear magnetic resonance image detection apparati for predetermined body parts, a considerable part of the body remains outside the detection cavity and acts as an antenna, through which electrical variations penetrate up to the detection area of the cavity, and particularly up to the body part under examination, and so up to the receiving coils.
The device object of the European patent 825 450 partly solves these problems since the shielding sleeve disclosed in the text is composed of a single conductive material fabric layer anchored to an outer fabric insulating layer made of synthetic fiber and to ensure contact with the body, the sleeve has a simple coulisse in which a fastening string is made to slide ensuring a perfectly closed form of the truncated cone-shaped sleeve but it does not completely solves the problem of the “antenna effect” performed by the part of the body that remains outside the apparatus since the electrical conductivity both of body-sleeve and sleeve-shielding connections of the apparatus and of the sleeve itself is relatively low.